3) \(đk:-x^2+9\ge0\Leftrightarrow x^2\le9\Leftrightarrow-3\le x\le3\)
10) \(đk:2x-3>0\Leftrightarrow x>\dfrac{3}{2}\)
6) \(đk:3x-9< 0\Leftrightarrow x< 3\)
21) \(đk:x\left(x+2\right)\ge0\)
\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x\ge0\\x+2\ge0\end{matrix}\right.\\\left\{{}\begin{matrix}x\le0\\x+2\le0\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x\ge0\\x\le-2\end{matrix}\right.\)
22) \(đk:9x^2-6x+1\ge0\Leftrightarrow\left(3x-1\right)^2\ge0\left(đúng\forall x\right)\)
3: ĐKXĐ: \(-3\le x\le3\)
10: ĐKXĐ: \(x>\dfrac{3}{2}\)
